Native C++ tests are not using CXXFLAGS

Native C++ tests are not using CXXFLAGS. This is mostly apparent on solaris, where CC and cc are quite different. Fixing this made a new warning appear on C++ tests for solstudio.

Also, when fixing this, it became apparent that the *FLAGS_TESTLIB/TESTEXE is just clumsy. In the long term, these super combined flags should be broken up. In the short term, just use *FLAGS_JDK explicitly, since that's what we're doing anyway.